Many building applications require electrical cables consisting of three insulated conductors and a bare ground conductor, preferably copper, all of which are surrounded by a non-metallic sheath or jacket. Underwriters Laboratories (UL) Standard 719, for example, defines such cables under the general designation “NM” for non-metallic sheathed cables, and the particular designation “NM-B” for building wire with a non-metallic sheath. The NM-B type of electrical cable is widely used in the building and construction industry, including for the new construction and remodeling of single-family housing and apartment-type residences.
There are at least two common NM and NM-B cable constructions using three insulated conductors and a bare ground. In what is typically known as a “round” construction, the inner insulated conductors and the associated bare ground conductor are “cabled” on a machine to produce a single inner construction in which all of the conductors are twisted together with a preferred lay-length and diameter. The bare ground wire is normally wrapped in a paper sheath, as can be the entire inner assembly of the twisted-together insulated and bare conductors. These “tabled” inner conductors are jacketed with a polyvinyl chloride (PVC) resin outer jacket to produce the finished cable.
Cabling gives the inner conductors a uniformly tight, rounded construction that holds up well for the final jacketing application. It also gives the finished cable a uniformly round appearance. Cabled inner conductors also inherently protect against possible movement, loosening, and ensuing damage of the individual conductors as they pass through capstans, dies, and paper folders during the various manufacturing phases. However, cabling the inner conductors disadvantageously adds labor-intensity and cost to the manufacturing process in comparison to other methods of construction.
The second typical type of construction of three conductor with bare ground NM and NM-B type electrical cables is the “bundled” construction. In the bundled construction, the inner insulated conductors and ground wires are not uniformly cabled. Instead the insulated conductors and bare grounding wires are randomly bundled, and sometimes intermittently twisted, in a quasi-rounded, or roughly square or diamond shaped fashion. A helical binder cord may or may not be used prior to the jacketing process. The bare ground wire and the entire inner construction are wrapped in paper before jacketing.
Bundling the inner conductors is more cost effective than cabling since certain aspects of the cabling process are eliminated during manufacture. However, bundling poses a number of other problems. During the jacketing, the individual conductors, which are not tightly cabled by twisting, can intermittently overlap or misalign. Additionally, the individual inner conductors can unpredictably and non-uniformly move and compete for space on capstans, in paper folders, or in dies, which can damage, crush, kink, or otherwise compromise the integrity of the current-carrying conductors and ground wires. The bundled inner conductors are also less uniform in appearance and assembly than cabled inner conductors.
Furthermore, the appearance of the outer finished jacket surrounding non-parallel, randomly bundled inner conductors can in turn be non-uniform and unpredictable. This can create problems during installation, since cables with inconsistently shaped outer jackets are difficult to pull through, under, or around joists, either alone or in conjunction with other wires. Bundle-constructed cables also create marketing and quality control problems, since production personnel and end users sometimes mistakenly associate an inconsistent non-uniform outer aesthetic of a finished product with manufacturing defects.
Hence, a need exists for new electrical cables and methods of their manufacture that address the foregoing problems with conventional cabled and bundled electrical cables with non-metallic sheaths. In addition to addressing these problems, such electrical cables and manufacturing methods should meet the requirements of the relevant NM and NM-B type electrical cable standards, particularly those related to electrical cables with three insulated conductors and a bare ground wire.